This invention relates to an external axial splint, or fixator, in particular for the dynamic repair of bone fractures.
For many years, it has become a practice in bone surgery to employ an external fixation device, essentially comprising a substantially rigid structure provided with bone-anchoring screws or bolts that are inserted in bone fragments on opposite sides of a given fracture, to encourage knitting of the bone by stabilizing the relative position of the involved bone fragments.
It has been noted that, with some types of fracture, knitting times can be accelerated and ultimate stability can be increased by exerting axial compression upon the bone fragments.
In the use of rigid external splints (fixators), muscular forces and/or external stresses are in general transmitted to the splint and not to the fractured bone. But, to encourage regeneration of hard bone, it is preferred that the patient apply some stress to the bone, to encourage more rapid and reliable knitting at the situs of the fracture. Conventional systems do not permit the application of such forces to the bone, on account of their relative rigidity and lack of deformability.
To overcome the problem of relative rigidity, U.S. Pat. No. 5,026,372 discloses a dynamic axial splint wherein an articulated parallelogram joint is the means of yieldably interconnecting spaced clamps that are respectively anchored to the separate fragments of a fractured bone. Specifically, the parallelogram joint comprises two spaced longitudinal members pivotally connected at their ends by two pairs of transverse links, thus allowing limited longitudinal displacement of one with respect to the other of the longitudinal members, when the joint is subjected to a compression force or a tension force.
A disadvantage of this known axial splint lies in the fact that, when the opposing members are subject to longitudinal displacement, they are also caused to move to some extent in a transverse direction, as governed by the accompanying angular displacement of the connecting links. Thus, the clamps which anchor the respective fragments of fractured bone lose their longitudinal alignment and have an adverse effect on the point of fracture. It has in fact been established that bony material which is undergoing consolidation has a very much lesser resistance to transverse (shear) forces than to bending or longitudinal forces. Thus, to contain the transverse movement, provision is made to limit relative longitudinal displacement of the longitudinal members to a few millimeters, but the indicated disadvantage is already appreciable with longitudinal displacements of a few tenths of a millimeter.
A further disadvantage of this known dynamic splint lies in the fact that, in order to repair or correct the fracture while the bony material is undergoing consolidation, the means for arresting dynamic movement must be released because of the particular configuration of the parallelogram joint and the manner in which it is anchored to opposing members fitted with clamps.
Another disadvantage of this known axial splint lies in its appreciable complexity and excessive cost due to the existence within the articulated parallelogram of a number of joints which among other things increase the risk of jamming at pivot connections.